multiplayer: support per-user camera views

[eanders-ms]

• For online multiplayer, support different camera views for each player.
• For local multiplayer, this would render as split view

This feature would allow players to individually roam a large tilemap.

TODO: define how a game should indicate it wants this behavior

• Is it a game setting/checkbox?
• Could it be implicit from the blocks present in the program?
• Should it be an explicit camera mode block, e.g. "multiplayer camera mode [same screen|own screens]"